Method for improving performance of hot air bulging forming workpiece

Abstract

The invention discloses a method for improving the performance of a hot air bulging forming workpiece. The method comprises the following steps that a pipe blank is pre-formed and then heated; the pipe blank is transferred to a lower die of a die, and gas is introduced into the pipe blank for pre-expansion; an upper die of the die is pressed downwards, and after die assembly, expansion continues to be conducted so that the peripheral surface of the pipe blank can be attached to a die cavity; and pressure maintaining and quenching are conducted, and the hot air bulging forming workpiece is obtained. According to the invention, the heated pipe blank is pre-expanded to a specified degree and then is closed, the fitting degree of the expanded pipe blank material and the mold is low before the mold is closed, the contact area is small, the temperature dissipation is slow, the pipe blank is kept at a relatively high temperature, the material fluidity is good, and the hot air expansion performance is good; after the mold is closed, expansion continues until the surface of the pipe blank is attached to the mold cavity for forming, deformation displacement is small, and the hot air expansion forming performance is high; due to the fact that certain air pressure exists in the pipe blank during mold closing, the problem that local depression is generated due to the fact that the section perimeter of the pipe blank is larger than that of the mold is solved, and the hot air expansion yield is increased.

Description

Technical Field

[0001] The present invention relates to the technical field of hot gas bulging, and particularly relates to a method for improving the performance of hot gas bulging formed workpieces. Background Art

[0002] With the rapid development of the automotive industry, consumers' demands for vehicle safety and lightweight are increasing day by day. As important load-bearing and safety components in the automotive structure, the hollow components of A-pillars, B-pillars and anti-collision beams not only need to have sufficient strength to resist collision impacts, but also need to minimize their own weight as much as possible, so as to improve the fuel or power consumption economy and environmental performance of the whole vehicle. For this reason, traditional manufacturing processes have been difficult to meet the high standards of modern automotive manufacturing, and the introduction of the hot gas bulging process provides an effective solution to this problem.

[0003] As a metal forming technology, the hot gas bulging process has the following advantages: a. High strength: Hot gas bulging involves heat treatment technology. After the material is heated to form austenite structure, it is formed by gas pressure, and then rapidly cooled to form a high-strength martensite structure. The strength of the final product material exceeds 1500 MPa; b. Light weight: Automotive lightweighting is the core means of energy conservation and emission reduction. Mileage has always been a pain point for new energy vehicles. Automotive lightweighting is an important solution strategy to improve mileage.

[0004] Although the hot gas bulging formed pipe fittings have the above advantages, for structures with curved surfaces and variable cross-sections (such as A-pillars), it is difficult for the conventional hot gas bulging process to maintain high formability. This is because the ultimate pressure of hot gas bulging generally does not exceed 70 MPa, which is one order of magnitude smaller than the pressure of internal high-pressure forming. This requires that during gas bulging forming, the tube blank should maintain a high temperature so that the material has good fluidity to meet the forming requirements.

[0005] However, during the process of transferring the tube blank from the heating furnace to the forming die, heat exchange with the air and heat transfer to the die in a large area after die closing will inevitably cause heat dissipation, resulting in a lower temperature during gas bulging and making it difficult to expand and form. Among them, the heat transfer mode of contacting the die is the main reason for the temperature drop.

[0006] Patent CN220948195U discloses a vehicle A-pillar structure and a vehicle. The novel vehicle A-pillar structure is integrally formed by hot gas inflation of the A-pillar inner plate part and the A-pillar reinforcement plate part. Patent CN111203467A discloses a hot gas bulging and rapid cooling strengthening system for metal pipes. The process routes of the above patents are all that the pipe fittings are heated and then put into the mold cavity, and gas is filled into the pipe fittings after the mold cavity is closed to achieve gas bulging forming. However, since the pipe fittings are closely attached to the mold cavity after the mold is closed, the temperature drops rapidly, resulting in poor material fluidity when the air pressure is filled and high forming difficulty. In addition, for the case where the cross-sectional perimeter of the pipe blank is greater than the cross-sectional perimeter of the mold, the perimeter is forcibly compressed by the mold to make the product locally thickened (≤15%). Since there is no pressure support inside the pipe blank, local depression is likely to occur after the mold is closed.

[0007] Therefore, it is of great significance to research and develop an improved hot gas inflation process to reduce the temperature dissipation during the expansion of the pipe blank, avoid local depression and thus improve the forming rate of hot gas inflated pipe fittings. Summary of the Invention

[0008] In view of the deficiencies in the prior art, the present invention provides a method for improving the performance of hot gas inflated workpieces. Before closing the mold, the pipe blank is pre-inflated with gas. After inflation to a specified degree, the mold is closed, reducing the contact and fitting area between the pipe blank and the mold during inflation, thereby reducing temperature dissipation, ensuring a relatively high temperature during inflation, improving the forming performance, and increasing the qualified rate of hot gas inflated forming.

[0009] To solve the above technical problems, the first aspect of the present invention provides a method for improving the performance of hot gas inflated workpieces, including the following steps:

[0010] S1. After pre-forming the pipe blank, perform heat treatment;

[0011] S2. Transfer the pipe blank to the lower mold of the mold, and introduce gas into the pipe blank for pre-inflation;

[0012] Wherein, define the minimum cross-sectional perimeter of the hot gas inflated workpiece as A, the maximum cross-sectional perimeter as B, and pre-inflate the pipe blank until the cross-sectional perimeter x is (B + A) / 2 < x < A + 0.7*(B - A);

[0013] S3. Press down the upper mold of the mold. After closing the mold, continue to inflate the gas to make the outer peripheral surface of the pipe blank fit the mold cavity;

[0014] S4. Hold the pressure and quench to obtain the hot gas inflated workpiece.

[0015] The present invention pre-inflates a tube blank after heat treatment until it reaches a specified degree and then closes the mold. When pre-inflating before closing the mold, the fitting degree between the tube blank material and the mold is low, the contact area is small, and the temperature dissipation is slow. This enables the tube blank to maintain a relatively high temperature during pre-inflation, with good material fluidity and excellent hot gas inflation performance. After closing the mold, continue to inflate until the surface of the tube blank fits the mold cavity. The material deformation and displacement are small, and the hot gas inflation forming performance is good. Moreover, since the tube blank already contains a certain air pressure when closing the mold, it avoids the problem of local depression caused by the cross-sectional perimeter of the tube blank being larger than that of the mold, further improving the hot gas inflation forming performance and increasing the qualified rate of hot gas inflation.

[0016] Further, in S1, the preforming includes pre-bending and / or cross-section shaping.

[0017] Further, in S1, the temperature of the heat treatment is 20 - 50°C above the phase transformation temperature of the tube blank material, and the heat preservation time is 3 - 5 minutes.

[0018] Further, in S2, the pre-inflation is carried out until the air pressure inside the tube blank reaches 5 - 18 MPa, and the pre-inflation time is 1 - 3 seconds.

[0019] Further, in S2, the transfer process includes the process of placing the tube blank onto the mold and the process of positioning the tube blank in the lower mold cavity.

[0020] Further, after pre-inflation, the diameter of the tube blank ≤ the size of the mold in the orthogonal direction perpendicular to the closing direction of the cross-section along the central axis, to avoid the phenomenon of die biting during the mold closing process.

[0021] Further, in the absence of a draft angle, the direction with the largest diameter of the mold cross-section perpendicular to the central axis is used as the orthogonal direction of the mold closing direction.

[0022] Further, in S3, continue to inflate until the air pressure inside the tube blank reaches 20 - 30 MPa, and the time is 1 - 5 seconds.

[0023] Further, in S4, the pressure holding is to maintain the gas pressure inside the tube blank and the pressing force of the mold, and the pressure holding time is 12 - 20 seconds.

[0024] Further, in S4, the cooling rate of the quenching is 40 - 50°C / s.

[0025] Further, in S1, the material of the tube blank is 22MnB5, and the heat treatment temperature is 930 - 950°C.

[0026] Further, in S1, the tube blank is a seamless round tube or a welded round tube.

[0027] The second aspect of the present invention provides a hot gas inflation formed workpiece prepared by the method described in the first aspect.

[0028] The beneficial effects of the present invention:

[0029] In the present invention, the pre-expanded tube blank after heat treatment is pre-expanded to a specified degree and then the mold is closed. When the tube blank is pre-expanded before closing the mold, the degree of fit between the tube blank material and the mold is low, the contact area is small, and the temperature dissipation is slow. As a result, the tube blank can maintain a relatively high temperature during pre-expansion, the material has good fluidity, and the hot gas expansion performance is good.

[0030] In the present invention, after the mold is closed, the tube blank is continuously expanded until the surface of the tube blank fits and forms with the mold cavity. The deformation displacement is small, the hot gas expansion forming performance is high, and since there is a certain air pressure inside the tube blank when the mold is closed, the problem of local depression caused by the cross-sectional perimeter of the tube blank being greater than the cross-sectional perimeter of the mold is avoided, further improving the hot gas expansion forming performance and increasing the yield rate of hot gas expansion. Brief Description of the Drawings

[0031] In order to more clearly illustrate the technical solutions of the present invention, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0032] Figure 1 is a schematic diagram of the gas expansion forming device adopted by the hot gas expansion method of the present invention;

[0033] Figure 2 is a flowchart of the method for improving the performance of the hot gas expansion formed workpiece of the present invention;

[0034] Figure 3 is a schematic diagram of the contact situation between the tube blank and the mold before closing the mold of the present invention;

[0035] Figure 4 is a schematic diagram of the contact situation between the tube blank and the mold after closing the mold of the present invention;

[0036] Figure 5 is a schematic diagram of the local depression situation during mold closing caused by the excessive cross-sectional perimeter of the tube blank;

[0037] Figure 6 is a simulation diagram of the workpiece thinning in Example 1;

[0038] Figure 7 is a simulation diagram of the workpiece thinning in Comparative Example 1;

[0039] Figure 8 is a simulation diagram of the martensite conversion rate of the workpiece in Example 1;

[0040] Figure 9 is a simulation diagram of the martensite conversion rate of the workpiece in Comparative Example 1;

[0041] Explanation of the reference numerals in the drawings: 1. Upper mold, 2. Lower mold, 3. Tube blank, 4. Punch, 5. Gas source pipeline, 6. Cooling water circuit. Specific Embodiment

[0042] The technical solution of the present invention will be clearly and completely described below in conjunction with specific embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts belong to the scope of protection of the present invention.

[0043] This embodiment provides a method for improving the performance of a hot gas bulging formed workpiece, using Figure 1 the shown hot gas bulging forming device, which includes the upper die 1 and the lower die 2 of the mold. When the mold is closed, the tube blank 3 is placed between the upper die 1 and the lower die 2. Both ends of the tube blank 3 are in contact connection with the punch 4, and a gas source pipeline 5 is arranged in the middle of one of the punches 4 for injecting gas into the tube blank 3. Cooling water channels 6 are arranged in both the upper die 1 and the lower die 2.

[0044] Refer to Figure 2 , the method for improving the performance of the hot gas bulging formed workpiece in this embodiment includes the following steps:

[0045] S1. Perform pre-forming and heat treatment on the tube blank;

[0046] S2. Transfer the tube blank to the lower die of the mold and inject gas into the tube blank for pre-bulging. Refer to Figure 3 , at this time, the tube blank 3 only contacts the lower die 2 and does not contact the upper die 1, with less temperature dissipation. During pre-bulging, it maintains a relatively high temperature, has large fluidity, and can achieve a large expansion; in addition, because the mold is not closed, the tube blank has a high degree of freedom. To prevent the tube blank from moving during bulging, both ends of the tube blank are fixed by a blank holder;

[0047] Among them, define the minimum cross-sectional perimeter of the hot gas bulging formed workpiece as A, the maximum cross-sectional perimeter as B, and pre-bulge the tube blank until the cross-sectional perimeter x is (B + A) / 2 < x < A + 0.7*(B - A);

[0048] S3. Press down the upper die of the mold to close the mold. Refer to Figure 4 , at this time, the tube blank 3 contacts both the upper die 1 and the lower die 2. Although the large contact area causes temperature drop, due to the previous pre-bulging, the deformation displacement of the tube blank is small at this time. Continuing to inject gas makes the outer peripheral surface of the tube blank fit the mold cavity; when the mold is closed, there is a certain air pressure inside the tube blank, which can avoid the problem of local depression caused by the cross-sectional perimeter of the tube blank being greater than the cross-sectional perimeter of the mold as shown in Figure 5 ;

[0049] S4. Hold the pressure and quench to obtain the hot gas bulging formed workpiece. By holding the pressure, the material springback is reduced and the material at the thicker wall position is evenly distributed to the thinner wall position to a certain extent, which is beneficial to the uniform wall thickness of the product. By introducing cooling water into the cooling water channel in the mold, the part is quickly cooled to below 200 °C.

[0050] In this embodiment, the tube blank after heat treatment is pre-inflated until it reaches a specified degree and then the mold is closed. When pre-inflating before closing the mold, the fitting degree between the tube blank material and the mold is low, the contact area is small, and the temperature dissipation is slow. As a result, the tube blank can maintain a relatively high temperature during inflation, the material has good fluidity, and the hot gas bulging performance is good. After closing the mold, continue to inflate until the surface of the tube blank fits the mold cavity. The deformation displacement is small, the forming performance is high, and since there is already a certain air pressure inside the tube blank when closing the mold, the problem of local depression caused by the cross-sectional perimeter of the tube blank being larger than the cross-sectional perimeter of the mold is avoided, further improving the hot gas bulging forming performance and increasing the qualified rate of hot gas bulging.

[0051] As a preferred embodiment, in S1, the tube blank is a seamless round tube or a welded round tube; the preforming includes pre-bending and / or cross-section shaping; the temperature of the heat treatment is 20 - 50 °C above the phase transformation temperature of the tube blank material, and keep the temperature for 3 - 5 min. Specifically, the material of the tube blank is 22MnB5, and the heat treatment temperature is 930 - 950 °C.

[0052] As a preferred embodiment, in S2, the transfer process includes the process of placing the tube blank on the mold and the process of positioning the tube blank in the lower mold cavity; pre-inflate until the air pressure in the tube blank is 5 - 18 MPa, and the pre-inflation time is 1 - 3 s.

[0053] As a preferred embodiment, after pre-inflation, the diameter of the tube blank ≤ the dimension in the orthogonal direction of the mold cross-section perpendicular to the central axis in the mold closing direction. When there is no draft angle, the direction with the largest diameter of the mold cross-section perpendicular to the central axis is used as the orthogonal direction of the mold closing direction, so as to avoid the phenomenon of die biting during the mold closing process.

[0054] As a preferred embodiment, in S3, continue to inflate until the air pressure in the tube blank is 20 - 30 MPa, and the time is 1 - 5 s.

[0055] As a preferred embodiment, in S4, the pressure holding is to maintain the gas pressure in the tube blank and the pressing force of the mold, and the pressure holding time is 12 - 20 s; the cooling rate of the quenching is 40 - 50 °C / s.

[0056] Another embodiment provides a hot gas bulging formed workpiece prepared by the method described in the above embodiment.

[0057] Taking a 22MnB5 round tube with a diameter of 33.5 mm as the original billet, an A-pillar product with a cross-sectional change rate of 8% and a wall thickness of 1.6 mm is formed by hot gas bulging. The minimum cross-sectional perimeter is A = 33.86 mm, and the maximum cross-sectional perimeter is B = 36.55 mm.

[0058] Example 1

[0059] This example relates to a method for improving the performance of hot gas bulging workpieces, including the following steps:

[0060] (1) Heat the billet to 930 - 950 °C and hold for 3 min;

[0061] (2) Transfer the billet to the lower die and introduce gas into the billet for pre-bulging. It linearly rises to 15 MPa in 2 s. At this time, the cross-sectional perimeter x of the billet is 35.5 mm;

[0062] (3) Press down the upper die of the mold. After closing the mold, continue to bulge for 1 s to 20 MPa to make the outer peripheral surface of the billet fit the mold cavity;

[0063] (4) Hold the pressure for 12 s and quench.

[0064] Comparative Example 1

[0065] This comparative example relates to a hot gas bulging method, including the following steps:

[0066] (1) Heat the billet to 930 - 950 °C and hold for 3 min;

[0067] (2) Transfer the billet to the lower die and close the mold to bulge. It linearly rises to 35 MPa in 3 s to make the outer peripheral surface of the billet fit the mold cavity;

[0068] (4) Hold the pressure for 12 s and quench.

[0069] The thinning simulation results of the hot gas bulging workpieces in Example 1 and Comparative Example 1 are as Figure 6 - 7 shown. It can be seen that the overall thinning in Example 1 is more uniform. The maximum thinning rate is shown in Table 1. The maximum thinning rate in Example 1 is only 12%, far lower than 20% in the comparative example, and the uniformity is better.

[0070] The martensite conversion rate simulation results of the hot gas bulging workpieces in Example 1 and Comparative Example 1 are as Figure 8 - 9 shown. It can be seen that the martensite conversion rate in Example 1 is better. The specific parameters are shown in Table 1. The martensite conversion rate in Example 1 is 100%, while the lowest martensite conversion rate in Comparative Example 1 is only about 61%, resulting in incomplete martensite conversion and affecting the performance of the workpiece.

[0071] Table 1

[0072] Mold sticking rate Maximum thinning rate Lowest martensite conversion rate Example 1 100％ 12％ 100％ Comparative Example 1 95％ 20.8％ 61％

[0073] In addition, as can be seen from Table 1, the film sticking rate of Example 1 is better, that is, the formed shape of the workpiece obtained in Example 1 is better, while there is a problem that the local forming in Comparative Example 1 is not in place.

[0074] In summary, in the present invention, the pre-expanded tube blank after heat treatment is pre-expanded to a specified degree and then clamped. When pre-expanding before clamping, the fitting degree between the tube blank material and the mold is low, the contact area is small, and the temperature dissipation is slow, so that the tube blank can maintain a relatively high temperature during expansion, the material has good fluidity, and the hot gas expansion performance is good; after clamping, continue to expand until the surface of the tube blank fits with the mold cavity to form, with small deformation displacement and high hot gas expansion forming performance. And because there is a certain air pressure inside the tube blank when clamping, the problem of local depression caused by the cross-sectional perimeter of the tube blank being greater than the cross-sectional perimeter of the mold is avoided, further improving the hot gas expansion forming performance and the qualified rate of hot gas expansion.

[0075] In summary, in the present invention, the pre-expanded tube blank after heat treatment is pre-expanded to a specified degree and then clamped. When pre-expanding before clamping, the fitting degree between the tube blank material and the mold is low, the contact area is small, and the temperature dissipation is slow, so that the tube blank can maintain a relatively high temperature during expansion, the material has good fluidity, and the hot gas expansion performance is good. In the present invention, after clamping, continue to expand until the surface of the tube blank fits with the mold cavity to form. Because there is a certain air pressure inside the tube blank when clamping, the problem of local depression caused by the cross-sectional perimeter of the tube blank being greater than the cross-sectional perimeter of the mold is avoided, further improving the hot gas expansion forming performance and the qualified rate of hot gas expansion.

[0076] The present invention has been described in detail above in combination with specific embodiments and exemplary examples, but these descriptions should not be construed as limiting the present invention. Those skilled in the art understand that without departing from the spirit and scope of the present invention, various equivalent substitutions, modifications or improvements can be made to the technical solutions and their implementation manners of the present invention, and these all fall within the scope of the present invention. The protection scope of the present invention is subject to the appended claims.

Claims

1. A method for improving the performance of a hot air bulging workpiece, characterized in that: The steps include: S1, pre-forming and then heating the tube blank; S2, transferring the tube blank to the lower die of the mold, and introducing gas into the tube blank for pre-inflation; The minimum cross-sectional perimeter of the hot air bulging workpiece perpendicular to the central axis is defined as A, the maximum cross-sectional perimeter is defined as B, and the cross-sectional perimeter x of the pre-bulged tube is defined as (B+A) / 2. <x<A+0.7*(B-A); S3, press the upper mold of the mold downward, and continue to inflate after the mold is closed so that the outer peripheral surface of the tube blank fits the mold cavity; S4, maintaining pressure and quenching to obtain the hot air bulging formed workpiece.

2. The method for improving the performance of a hot air bulging workpiece according to claim 1, characterized in that: In S1, the preforming includes pre-bending and / or cross-section shaping.

3. The method for improving the performance of a hot air bulging workpiece according to claim 1, characterized in that: In S1, the temperature of the heating treatment is 20-50°C above the phase change temperature of the tube blank material, and the temperature is kept for 3-5 minutes.

4. The method for improving the performance of a hot air bulging workpiece according to claim 1, characterized in that: In S2, the pre-inflation is performed until the air pressure inside the tube is 5-18 MPa, and the pre-inflation time is 1-3 s.

5. The method for improving the performance of a hot air bulging workpiece according to claim 1, characterized in that: In S3, the inflation is continued until the air pressure in the tube reaches 20-30 MPa, and the time is 1-5 s.

6. The method for improving the performance of a hot air bulging workpiece according to claim 1, characterized in that: In S4, the pressure holding is to maintain the gas pressure in the tube blank and the pressing force of the mold, and the pressure holding time is 12-20s.

7. The method for improving the performance of a hot air bulging workpiece according to claim 1, characterized in that: In S4, the cooling rate of the quenching is 40-50°C / s.

8. The method for improving the performance of a hot air bulging workpiece according to claim 1, characterized in that: In S1, the tube blank is made of 22MnB5 and the heating treatment temperature is 930-950°C.

9. The method for improving the performance of a hot air bulging workpiece according to claim 1, characterized in that: In S1, the tube blank is a seamless round tube or a welded round tube.

10. A hot air bulging workpiece prepared by the method according to any one of claims 1 to 9.

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